Re: RFA: tolerate unavailable struct return values

[Elena Zannoni <ezannoni at cygnus dot com>]

Michael Snyder writes:
 > Daniel Jacobowitz wrote:
 > > 
 > > On Fri, Nov 30, 2001 at 03:49:52PM -0500, Jim Blandy wrote:
 > > >
 > > > Daniel Jacobowitz <drow@mvista.com> writes:
 > > > > On Thu, Nov 29, 2001 at 05:09:13PM -0500, Jim Blandy wrote:
 > > > > >
 > > > > > On some architectures, it's impossible for GDB to find structs
 > > > > > returned by value.  These shouldn't be failures.  Should they be
 > > > > > passes?
 > > > >
 > > > > Out of curiousity, which architectures?  And to be pedantic, I suspect
 > > > > that it might be "not always possible" rather than actually
 > > > > impossible.
 > > >
 > > > The one I have in mind is the S/390, although I'm pretty sure there
 > > > are others.  I've included the bug report I sent to the S/390 GCC
 > > > maintainers below.
 > > >
 > > > One approach would be to hope that the return buffer's address was
 > > > still there in the register it was passed in.  But there's no way to
 > > > tell when you're wrong.  GDB will just print garbage, and the user
 > > > will think their program is wrong.  Better to simply say, "I can't
 > > > find this information reliably", and let the user, who knows their
 > > > program, find another way to get the info --- setting a breakpoint on
 > > > the return statement, or looking at where the caller put the
 > > > structure.
 > > 
 > > Hmmmm.  I wonder if MIPS could ever be affected by this?  I don't think
 > > the MIPS ABI specifies that $a0 remains live.  It looks as if the value
 > > of $a0 is always returned in $v0 in such functions, though.
 > 
 > It's not an uncommon problem, and I imagine we get it wrong a lot of the time.

Have you looked at the macro VALUE_RETURNED_FROM_STACK ? I defined that
long time ago for hppa. It looks like the rs6000-tdep.c tries to deal
with the same problem as well.

Maybe we should clean up that code, which came in as part of the HP
merge :-(.

Elena